1. Field of the Invention
This invention relates to electrical devices comprising conductive polymer compositions and methods for making such devices.
2. Introduction to the Invention
Electrical devices comprising conductive polymer compositions are well-known. Such compositions comprise a polymeric component and, dispersed therein, a particulate conductive filler such as carbon black or metal. Conductive polymer compositions are described in U.S. Pat. No. 4,237,441 (van Konynenburg et al), U.S. Pat. No. 4,388,607 (Toy et al), U.S. Pat. No. 4,534,889 (van Konynenburg et al), U.S. Pat. No. 4,545,926 (Fouts et al), U.S. Pat. No. 4,560,498 (Horsma et al), U.S. Pat. No. 4,591,700 (Sopory), U.S. Pat. No. 4,724,417 (Au et al), U.S. Pat. No. 4,774,024 (Deep et al), U.S. Pat. No. 4,935,156 (van Konynenburg et al), U.S. Pat. No. 5,049,850 (Evans et al), U.S. Pat. No. 5,250,228 (Baigrie et al), and U.S. Pat. No. 5,378,407 (Chandler et al), and in pending U.S. application Ser. Nos. 08/085,859 (Chu et al, filed Jun. 29, 1993) now U.S. Pat. No. 5,451,919, Ser. No. 08/255,497 (Chu et al, filed Jun. 8, 1994) now U.S. Pat. No. 5,582,770, and Ser. No. 08/408,769 (Wartenberg et al, filed Mar. U.S. Pat. No. 22, 1995), abandoned in favor of continuation application Ser. No. 08/789,962, filed Jan. 30, 1997, now U.S. Pat. No. 5,747,147, issued May 5, 1998. The disclosure of each of these patents and applications is incorporated herein by reference. These compositions often exhibit positive temperature coefficient (PTC) behavior, i.e. they increase in resistivity in response to an increase in temperature, generally over a relatively small temperature range. The size of this increase in resistivity is the PTC anomaly height.
PTC conductive polymer compositions are particularly suitable for use in electrical devices such as circuit protection devices that respond to changes in ambient temperature and/or current conditions. Under normal conditions, the circuit protection device remains in a low temperature, low resistance state in series with a load in an electrical circuit. When exposed to an overcurrent or overtemperature condition, however, the device increases in resistance, effectively shutting down the current flow to the load in the circuit. For many applications it is desirable that the device have as low a resistance and as high a PTC anomaly as possible. The low resistance means that there is little contribution to the resistance of the electrical circuit during normal operation. The high PTC anomaly allows the device to withstand the applied voltage. Although low resistance devices can be made by changing dimensions, e.g. making the distance between the electrodes very small or the device area very large, the most common technique is to use a composition that has a low resistivity. The resistivity of a conductive polymer composition can be decreased by adding more conductive filler, but this generally reduces the PTC anomaly. A possible explanation for the reduction of the PTC anomaly is that the addition of more conductive filler (a) decreases the amount of crystalline polymer that contributes to the PTC anomaly, or (b) physically reinforces the polymeric component and thus decreases the expansion at the melting temperature. It is, therefore, often difficult to achieve both low resistivity and high PTC anomaly.